diff --git a/src/authentic2_pratic/models.py b/src/authentic2_pratic/models.py
index a7b77ed..a647637 100644
--- a/src/authentic2_pratic/models.py
+++ b/src/authentic2_pratic/models.py
@@ -343,17 +343,20 @@ class ServiceInstance(Model):
         return (self.slug, self.service.natural_key(), self.collectivity.natural_key())
 
     def clean(self):
-        if self.collectivity and self.service and self.service.is_global:
-            qs = ServiceInstance.objects.exclude(id=self.id)
-            qs = qs.filter(collectivity=self.collectivity,
-                    service=self.service)
-            if qs.exists():
-                raise ValidationError(_('There can be only one instance of a global service by collectivity'))
-        if self.service and self.service.is_global:
-            self.service_url = self.service.service_url
-            self.metadata_url = self.service.metadata_url
-        if not self.service.is_global and not self.service_url:
-            raise ValidationError(_('Service URL field is required'))
+        super(ServiceInstance, self).clean()
+        if self.service_id:
+            if self.collectivity and self.service.is_global:
+                qs = ServiceInstance.objects.exclude(id=self.id)
+                qs = qs.filter(collectivity=self.collectivity,
+                        service=self.service)
+                if qs.exists():
+                    raise ValidationError(_('There can be only one instance of a global service by collectivity'))
+            if self.service.is_global:
+                self.service_url = self.service.service_url
+                self.metadata_url = self.service.metadata_url
+                self.cas_service_url = self.service.cas_service_url
+            if not self.service.is_global and not self.service_url:
+                raise ValidationError(_('Service URL field is required'))
 
     class Meta:
         verbose_name = _('service instance')